since a short time (hadn't touched the templates for quite a while), I cannot access all templates anymore, even though they have the same lime owner, lime permissions, unix owner/group and file permissions as one that is accessible. What else could it be?

Important: By "accessible" I mean access through the template editor. All templates seem to work flawlessly in productive surveys.

We updated to 2.05+ b140703 recently. The problem was also reproducable using the "admin" user.

If it was a really old version, than the problem could be the following: I think in 1.9x the location of the templates that have not been shipped with limesurvey are not located in the template folder, but in the folder \upload\templates.

Those files can also not be modified and need to be copied to the new folder. Only then they can be modified. Have a look it this is the problem.